This invention relates to improvements in holographic storage materials and more particularly in lithium niobate in the form of a single crystal that is one of the holographic storage materials.
It is well known that, by splitting a beam of laser light into an object beam of light and a reference beam of light and superposing wave surfaces for both beams of light, the interference of light occurs in all that space in which the object and reference beams of light exist at the same time. If a body of photosensitive material or holographic storage material having a thickness exceeding a certain magnitude is placed in such a space then an interference pattern developed in that portion of the space in which the body of holographic storage material is placed is recorded on the body of holographic storage material as it stands. The interference pattern recorded on the holographic storage body is called a three dimensional hologram. Three dimensional holograms are one type of three dimensional diffraction gratings and are operative to diffract a beam of light incident thereupon strictly following the Bragg's condition of diffraction. Therefore it is possible to record on a single body of holographic storage material a plurality of holographs formed of beams of light having different wavelengths as well as independently reproducing the indivdual holograms from the holographic storage body. This is true in the case of a change in the orientation of the holographic body relative to the associated interference pattern of laser light. Accordingly three dimensional holograms are recently in the limetight as new recording means for use with computers and a future development thereof can be expected because they are latently high in storage capability.
One of materials recently watched as the three dimentional holographic storage materials is monocrystalline lithium niobate (LiNbO.sub.3). The single crystal of that material records an interference pattern of laser light thereon as an optical damage, that is, a local change in an index of refraction thereof and is advantageous in that it has both a high storage capability and a high efficiency of diffraction while the re-recording is possible. Further it is less sensitive to temperatures in the range of room temperature, and chemically stable. In view of the standpoint of the wavelength, output and stability of laser light and also in view of the standpoint of the physical properties of the monocrystalline lithium niobate a beam of laser light caused from argon and having a wavelength of 4880A is presently used as a source of laser light for recording. For the present that beam of laser light from argon is disadvantageous in that it is low in a light energy required to provide a diffraction efficiency of 20%. Such a light energy is called hereinafter a sensitivity of recording. The figure of 20% just specified for the diffraction efficiency results from the fact that holographic storage materials having the diffraction coefficient in the order of such a figure can be put to practical use.
It is accordingly an object of the present invention to provide an improved holographic storage material for use in three dimensional holograms high in both recording sensitivity and efficiency of saturation diffraction by adding lithium niobate in the form of a single crystal with either iridium or uranium.